As a conventional method of manufacturing a polyacetal copolymer, the cation copolymerization is known in which trioxane and a cyclic ether and/or cyclic formal having at least one carbon-carbon bond are used as a main monomer and a comonomer, respectively. Cationic active catalysts used for the above copolymerization may include Lewis acid, in particular, halides of boron, tin, titanium, phosphorus, arsenic and antimony, for example, boron trifluoride, tin tetrachloride, titanium tetrachloride, phosphorus pentachloride, phosphorus pentafluoride, arsenic pentafluoride and antimony pentafluoride, and compounds such as complexes or salts thereof: protonic acid, for example, perchloric acid; esters of protonic acid, in particular, esters of perchloric acid and lower aliphatic alcohol, for example, tert-butyl perchlorate ester; anhydrides of protonic acid, in particular, mixed anhydrides of perchloric acid and lower aliphatic carboxylic acid, for example, acetyl perchlorate, or alternatively trimethyloxonium hexafluorophosphate, triphenyl-methylhexafluoro alzenate, acetyl tetrafluoroborate, acetyl hexafluorophosphate, acetyl hexafluoro alzenate and the like. Among these, boron trifluoride, or coordination complexes of boron trifluoride and organic compounds, for example, ethers are the most common catalysts for polymerization in which trioxane is used as a main monomer, and they are widely used in industries.
However, in the case of polymerization catalysts commonly used, such as a boron trifluoride-based compound, the polymerization rate may decrease rapidly in the late stage of polymerization, and thus the polymerization conversion percentage can hardly reach near 100% in a short time, resulting in a very time-consuming and inefficient process. Further, decomposition of a polymer product promoted by the action of a catalyst may become relatively favored in the late stage of polymerization, resulting in decreased molecular weights and even deteriorated qualities such as thermal stability. Further, increasing the amount of a polymerization catalyst may generally promote the polymerization rate and also improve the polymerization convention percentage, but is not necessarily preferred in view of the overall efficiency of the manufacturing process because the quality of a crude polymer product may be increasingly deteriorated, requiring complicated stabilization treatment in the subsequent steps.
Accordingly, the following approach is widely used: a solution comprising a catalyst-deactivating agent is added at a stage of a relatively low polymerization percentage to quench polymerization, and remaining unreacted monomers are washed, recovered and purified for recycling.
Further, a plurality of approaches for improving a polymerization conversion percentage have also been proposed, comprising: improving a polymerization machine; and improving a method of supplying a catalyst. For example, proposed are an approach in which a polymerization machine is inclined by 1 to 10° to improve a polymerization conversion percentage per equipment (Patent Document 1) and an approach in which a weir is provided at a discharge outlet of a polymerization machine (Patent Document 2) as well as methods in which a catalyst and a comonomer are pre-mixed, and then added to trioxane (Patent Documents 3 and 4). These approaches are each effective for increasing a polymerization conversion percentage when a boron trifluoride-based polymerization catalyst is used.
Further, an approach has also been proposed in which unreacted monomers are directly recovered for recycling without deactivating and washing a crude polymer at the late stage of polymerization by virtue of a highly-active and nonvolatile polymerization catalyst (Patent Document 5). According to the above approach, monomers can be recovered directly from a crude polymer before deactivation, which is difficult to achieve when the conventional boron trifluoride systems are used. Further, side reactions also tend not to occur at a stage where a high conversion percentage is reached as compared with a case where a boron trifluoride-based polymerization catalyst is used. Therefore, a crude polymer containing a less amount of unreacted monomers and having excellent thermal stability can be obtained via a very simple process.
Patent Document 1: Japanese Examined Patent Application Publication No. H05-008725
Patent Document 2: PCT International Publication No. WO 1996/13534
Patent Document 3: Japanese Unexamined Patent Application Publication No. H11-255854
Patent Document 4: Japanese Unexamined Patent Application Publication No. H11-124422
Patent Document 5: Japanese Unexamined Patent Application Publication No. H09-278852